Method of firing a tunnel kiln



Sept 1966 D. E. TOMKINS 3,271,487

METHOD OF FIRING A TUNNEL KILN Filed March 13, 1961 4 Sheets-Sheet l INVENTOR.

DAVID E. TOMKINS Attorneys Sept. 6, 1966 r. E. TOMKINS 3,271,437

METHOD OF FIRING A TUNNEL KILN Filed March 13, 1961 4 Sheets-Sheet 2 1 1 I I v I I 1 1 I I r I 2 INVENTOR. DAVID E. TO/VIKINS 2 6 flax/ m Attorneys United States Patent 3,271,487 METHOD OF FIRING A TUNNEL lKlLN David E. Tomkius, New Castle, Pa, assignor to Shenango China, Inc., New Castle, 1%., a corporation of Pennsylvania Filed Mar. 13, 1961, Ser. No. 95,192 2 Claims. (Cl. 264--64l) This application is a continuation-in-part of my prior application for Tunnel Kiln and Continuous Conveyor System, Serial No. 729,778, now Patent Number 2,974,387, issued March 14, 1961, filed April 21, 1958.

The present invention relates generally to apparatus designed for use in the firing of ceramic articles.

More particularly, the invention is concerned with the provision of an improved continuous kiln in the form of a straight tunnel and methods of kiln operation which are adapted for the bisque or glost firing of ceramic ware or th production of ceramic ware by a single fire.

As is well known, a conventional type of pottery kiln used for automated Ware manufacture is one in which the ware is loaded single-high on horizontal, refractory tables of kiln cars which are sequentially run through the firing and cooling zones of the kiln. customarily, these kilns are top fired by burning gas or oil, or by positioning electrical resistances, above and at the sides of the path of travel of the ware.

When the firing procedure described above is used in the bisque firing of ware, the biscuit frequently cracks because of uneven heating. This cracking of unevenly heated ware has been found to be caused primarily by the abrupt volume expansion occurring as a result of quartz inversion, which is the change in crystal formation when the quartz in the clay passes from the alpha to the beta phase. If the biscuit temperature is substantially uniform, the change in crystal formation of the quartz presents no problem since the piece will be wholly within either the alpha :or beta phase at any given time. However, if the piece is heated unevenly, such as occurs in side and top fired kilns wherein the rim of the piece is hotter than the relatively thicker foot portion, the piece will, in the course of firing, be partially in the alpha phase and partially in the beta phase. The resulting volume expansion of the rim relative to the foot and center when the piece is heated, or the con traction of the rim when the piece is cooled, produces internal stresses which cause the ware to crack.

The firing of the ware, as practiced in conventional kilns, has also been found to be the principal cause of spit-out, pimples, bubbles, and similar defects which occur in the glost firing of vitrified ware and in the firing of semi-vitrified ware. Most of the gas which causes these defects evolve from the organics in the colors of the decoration and in the glaze, from reaction of the glaze with the bisque body, and from gas occluded in the dry, unfired glaze; in conventional kilns, the glaze frequently melts and commences to mature before such gases can escape. Similarly, in the production of semi-vitrified ware, the glaze often melts before gas occluded in the green ware can escape.

Another disadvantage of conventional kiln construction and operation is that the ware must be heated relatively slowly to its firing temperature and, likewise,

327L487 Patented Sept. 6, 1966 cooled relatively slowly. This has always been done heretofore to avoid cracking or dunting due to the thermal stresses and strains of sudden changes in temperature. Another factor contributing to the time required for the manufacture of ceramic ware is the conventional practice of first loading the ware in kiln furniture, which is then loaded on kiln cars for passage of the ware through the kiln. After firing, the kiln furniture must be unloaded from the cars and, later, in a separate operation, the ware is removed from the saggers or other kiln furniture.

The present invention overcomes the foregoing problems and disadvantages of conventional pottery kiln construction and operation; it provides a method and ap paratus for commercially producing ceramic ware whereby the ware is quickly taken to its firing temperature and, after a due soaking period, rapidly cooled so that the Ware may then be taken immediately from the kiln to succeeding operations, such as tumbling and cleaning preparatory to glazing, in the case of bisque, and for foot polishing, decoration, and the like, in the case of glazed ware. Loading and unloading of the ware in kiln furniture is not required. Further, in addition to minimizing cracking or dunting, and spit-out or like defects, the total firing time is lessened, resulting in greater thermal efficiency. Due to the shorter firing time in the glost fire, coloring under the glaze is clearer and sharper, due to lesser solution of the color ingredients in the glaze. By minimizing the cause of spit-out, thicker coatings of glaze can be fired.

By controlling the heat in zones along the path of travel of the kiln cars, and by controlling the heat into zones above and below the platforms, it is possible to uniformly heat the ware during the bisque fire so that the quartz contained in the clay will at all times be wholly in either the alpha or beta phases, thus avoiding cracking of the biscuit clue to differential volume expansion. Also, it is possible to drive off the gases contained in the coloring and in the biscuit during glost firing before the glaze matures. Similarly, the occluded gases in the green clay can be driven off in the firing of semi-vitrified ware before the glaze melts, thus avoiding the blow holes and pimples which occur when ceramic kilns are merely top fired.

The bottom-up heating of the ware has the additional advantage of permitting the ware to be quickly brought to a high temperature, thus allowing more time for soaking and for cooling than heretofore possible. Because of the even, uniform heating of the ware, it can be heated rapidly 'without cracking from thermal strains and stresses.

Another feature is a method of kiln operation wherein the ware can be quickly and uniformly heated. More particularly, it is an object of the invention to provide a method of kiln operation in which the kiln is divided vertically and horizontally into a number of controllable heating zones.

Yet another feature of the invention is that all the above may be accomplished without the occurrence of impact loads on the tie rods, particularly during inversion of the tie rods shortly after their passage through the kiln.

The several features of the invention will be more clearly understood from the following description of one example of the invention.

In the accompanying drawings:

FIGURE 1 is a schematic, vertical sectional view of a kiln constructed according to the invention.

FIGURE 2 is a side elevation of one end of the straight tunnel kiln shown in FIGURE 1.

FIGURE 3 is a fragmentary sectional elevation, partially broken away, taken from the planes of line 3-3 of FIGURE 2.

FIGURE 4 is a further enlarged detail view taken from the plane of line 44 of FIGURE 3.

FIGURE 5 is a view taken from the plane of line 5-5 of FIGURE 4.

FIGURE 6 is a vertical sectional view taken in the plane of line 6-6 of FIGURE 7.

FIGURE 7 is a top elevation of a portion of the invention.

FIGURE 8 is a bottom elevation of the part of the invention shown in FIGURE 7.

FIGURES 9 and 10 are graphs illustrating a preferred mode of operation of the kiln comprising the invention; FIGURE 9 shows the relative operation of sets of top and bottom burners as ware, such as dinnerware, passes through an exemplary kiln of 150 feet in length in a period of 81 minutes; FIGURE 10 shows the increase and decrease in the temperature of the ware in such kiln during the same time and distance of travel.

There is provided according to the illustrated example of the invention a straight tunnel kiln It), lined with suitable refractory material, as best shown in FIGURES 1 and 3. The tunnel kiln 10 is supported by suitable frame members 11 and 12, and the mechanism of the tunnel kiln 10 may be shielded against dust and the like by a Wall 13. The tunnel of the kiln is indicated by the reference numeral l4. Along the bottom of the tunnel I4 is formed a slot 15. A number of tunnel cars 20 are entrained and are guided for movement in upright position along the slot and in inverted position along a return reach 22 immediately below the slot 15. The means for so guiding the cars 2% includes an upper railway comprising the rails 25 and 26 and a lower railway comprising the rails 27 and 28, the lower railway being located directly under the upper railway. The guide means also includes sprockets aligned with the rails of the railways at each end thereof and extending between the levels of the upper and lower railways respectively; for example, the sprocket 30 (see FIGURE 2) extends between the levels of the .upper and lower railways 25 and 27 and an additional sprocket 31 (see FIGURE 3) extends between the levels of the upper and lower railways 26 and 28. The sprockets 30 and 31 are mounted on a drive shaft 32, 32a which is connected to a power drive (not shown). The shaft 32, 32a is mounted in pillow blocks 33 for adjustment of the chain to be associated with the sprockets by means of the adjusting screws 34. The opposite end of the tunnel 10 (not shown) may exactly duplicate the structure described above, with the exception that the sprocket shaft may be simply an idler shaft and may be fixedly mounted, no pillow blocks 33 or adjusting screws 34 being required since adjustment from one end of the structure is all that is necessary. The chains which engage the sprockets 30 and 31 and the additional sprocket at the opposite ends of the tunnel comprise links 40, which are coupled to the outer ends of the wheel axles of the cars 20, as shown in FIGURE 3. It will be clear then that the chains, made up of the links 40, link together the cars and also extend along the length of the railway -23 and operatively engage with the sprockets 3t) and 31 and the corresponding sprockets at the opposite end of the kiln 10.

Each car 20 has a base portion which extends across the slot 15 when the car is positioned in the slot 15, such base portion, including a deck, being generally indicated by the reference numeral 42;. As shown in FIG. 3, the slot 15 is constructed so that portions of its side walls 4 overlie the sides of the base portion 42 with only a slight clearance 47 therebetween so that an effective radiant heat shield is formed along the sides of the car when in the tunnel 14. Similarly, the ends of the base portions of adjoining cars are formed with overlapping edge por' tions 46 so as also to form a radiant heat Shield.

The deck 42 of each kiln car is lined on its upper side with ceramic material 43, 44 and the deck has a metal lower side comprising metal portions 45. Metal sleeves 50 extend upwardly from such metal lower side to such ceramic upper side.

On each car 20 is kiln furniture projecting upwardly into the tunnel 14 when the car is in the slot 15. The furniture comprises a plurality of ceramic hollow columns 55 which are received within the metal sleeves 50 and are end abutted against the lowermost plate of the metal portions 45 (see FIGURE 3) and which extend upwardly through the ceramic upper side of the deck 42, that is, through the ceramic members 43 and 44, and into the tunnel 14. A ceramic platform 56 is supported at the top of the kiln associated with each car. The platform is provided with sockets 57 (see FIGURES 4 and 5) on its underside to loosely receive the columns 55.

Referring to FIGURES 6, 7, and 8, it will be seen that the platform 56 is formed on its underside with hollowed out portions 58, and on its upper surface with raised por tions 59 which may have the diamond configuration shown or may be ridges of any other suitable shape. The pur' pose of these hollowed-out portions 58 is to permit the platform to be rapidly heated in a manner to be hereinafter discussed in more detail, and, conversely, to be rapidly cooled after the ware is fired. The ware is supported on the ridges 59 so that a portion of the foot of each piece of Ware is exposed to radiant heat during firing, thus permitting the ware to be more uniformly heated than if it rested on a smooth, flat surface. Additionally, the ridges 59 stiffen the platform and provide strength to support the ware. Preferably, the platform is formed of a quartz-free, ceramic material so that it can be rapidly heated and cooled without cracking, since, as discussed above in connection with the bisque firing of ware, if the material forming the platform 56 contained an appreciable amount of quartz, a volume expansion would occur during the change in crystal formation of the quartz, thus inducing undesirable stresses in the platform.

Tie rods 60 extend through the columns 55 and are engaged at their upper ends with the platform 56. For example, the tie rods may have a T-shaped head 62 (see FIGURE 5) which is shielded from heat by a ceramic plug or insert 63. The T-shaped head is suspended from the shoulders s5 formed in the platform 56. The tie rods extend downwardly through the columns 55, being shrouded and protected thereby against the kiln fire dur ing the passage of the cars through the kiln tunnel 14, and the lower ends of the tie rods extend to the base portion of their associated car and to and through the metal lower side of their associated platform 42, that is, the metal tie rods extend through the base portion of their associated car, as most clearly seen in FIGURES 3 and 4. The rods 60 are tied to the base portion of their associated car by spring means such as the springs 61, such spring means urging the lower ends of the rods downwardly from the metal lower side of their base portion. It will thus be understood that the metal rods are maintained under spring-loaded tension, and that rod tension is relatively low when the cars 20 are upright and at all times when the rods are subjected to the elevated temperature within the tunnel 14, and that rod tension is relatively high only when the cars are inverted outside the tunnel as during the time their associated car passes along the return reach 22. It will be further understood that impact shocks are avoided as the tunnel cars 20 are inverted following their pass through the tunnel 14. In this respect the spring-loaded tie rod feature is to be contrasted to previously proposed kiln tie rods, such as that shown, for example, in US. Patent No. 1,895,408 to Davis. It is preferable that the spring means be almost expanded to zero deflection when the associated car is upright so that as each upright car passes through the tunnel 14, the rods 60 are subjected substantially only to the load of their own weight plus a very slight additional tension sufiicient to prevent the occurrence of any unsprung lost motion during car inversion. The spring rate and the amount of lost motion which is possible should-be sufficiently great that when the cars are inverted and the rods 60 support the full weight of the furniture, such full weight is entirely sprung.

In the illustrated embodiment of the invention, the kiln is gas fired, although it is apparent that it can be oil fired or heated electrically by suitably disposed resistances. The temperature within the tunnel 14 of the kiln is controlled in zones along the path of travel of the tunnel cars 20 as well as in separate zones above and below the platforms 56 of the cars. To this end the kiln is provided with a series of top burners '70 and a series of bottom burners 71 (see FIGURE 1). The top burners 70 are situated to over-fire the ware on the platforms 56 by radiant heat while the bottom burners 71 heat the ware from the bottom-up by convection and conduction.

The bottom and top burners are divided into groups which are regulated by separate controls (not shown) so that the temperature in each zone can be controlled as desired. The number of heat zones into which the kiln is divided by regulation of the burner controls, and the temperature of each zone, depends upon the type of operation, that is, whether the operation is either the bisque or glost firing of fully-vitrified ware, or the firing of semivitrified Ware.

As indicated in FIGURE 1, the tunnel is physically divided by a damper or bafile 75 into a heating section in which the top and bottom burners are located, and a cooling section which extends toward the exit end of the tunnel 14. The heating and cooling sections of the tunnel are generally designated by reference numerals 76 and 77, respectively. As shown, the bafile 75 is located ap' proximately /3 of the length of the kiln from the entrance end; however, the location of the baffle can be varied as desired to increase or decrease the relative lengths of the heating and cooling sections. It is also possible to use a plurality of bafiles if such use is found desirable. A draft of air is constantly created toward the entrance end of the kiln by a blower 79 located at the exit end of the tunnel 14 so as to prevent the hot gases in the heating section 76 from flowing into the cooling section '77, while suitable exhaust fines 78 are located in the roof of the kiln. The temperature control of the various heating zones in section 76 of the tunnel can be augmented by cooling holes (not shown) formed in the roof and side walls of the kiln.

FIGURES 9 and 10 graphically illustrate how a kiln constructed according to the invention may be operated for the bisque firing of ceramic ware. As shown, the heating section of the tunnel is divided into five temperature zones along the path of travel of the tunnel cars. In the first of these zones, the Ware is under-fired and heated rapidly to 1000 F., which is just under the quartz inversion temperature of approximately 1063.4 F. In, the second of the zones, the ware is slowly heated through the quartz inversion temperature to 1200 by bottom burners which are regulated by a second set of controls. After the ware has passed the critical quartz inversion point, it is again heated rapidly in a third zone to a soaking temperature of approximately 1800 F., and then is allowed to soak at this temperature in the fourth zone. In the third and fourth temperature zones, the ware is both top-fired and bottom-fired so that the heating can be accomplished as quickly as possible, the under-fire being etfectuated by the second set of bottom burners and the over-fire by separately controlled top burners in each zone. After the ware has soaked for the required length of time, the ware passes into the fifth temperature zone in which the ware cools in stages to 1000 F. This fifth zone is top-fired by two sets of separately controlled top burners. The end of the fifth temperature zone constitutes the end of the heating section of the tunnel, and from this point to the exit end of the kiln the ware is permitted to rapidly cool to room temperature so that it can be immediately removed for subsequent grinding operations and the like.

It will be apparent that, by the use of bottom and top burners which are separately controlled so as to divide the tunnel 14 of the kiln into a plurality of temperature zones, there is provided a system. for commercially producing ceramic Ware which avoids the defects which have heretofore been encountered. The features of bottomfiring so as to evenly heat the Ware through the quartz inversion temperature is to be contrasted from the conventional practice of side-firing which causes the ware to crack when the relatively thin rim portion of the ware is heated to the quartz inversion temperature before the thicker foot portion. It also will be apparent that the use of top and bottom burners permits the ware to be heated more rapidly than heretofore possible, thus permitting a longer soaking time to rid gases from the biscuit and coloring and a longer cooling time.

Although the method of kiln operation comprising a part of the invention has been specifically described in connection with bisque firing, it is to be understood that the same principles and advantages are applicable to glost firing and to the firing of semi-vitrified ware. For example, the bottom-firing in the first two temperature Zones which heats the ware from the bottom-up is effective to drive off the gases in the coloring and any occluded gas in the biscuit before the glaze melts at approximately 1200 F. In the case of semi-vitrified ware, the gases in the green clay are driven off in the first two temperature zones before the glaze matures.

In order to further increase the rapidity at which the ware may be heated, burners may be provided along the lower reach 22 of the tunnel cars. These burners are generally indicated by reference numeral in FIG- URE 1, and are shown as being located near the entrance of the kiln in a position to heat the platforms 56. By heating the platforms to approximately 300 B, it is possible to increase the temperature at the entrance of the tunnel 14 to at least 200 F., and, in this way, to materially reduce dunting of the Ware.

Other modifications and variations of the invention obviously will be apparent to those skilled in the art in the light of the above teachings. It is, therefore, to be understood that the invention is not to be limited to the embodiment shown and described for the purposes of disclosure, but is to include all variations and modifications within the spirit and scope of the invention as defined.

by the appended claims.

What is claimed is:

1. A method of firing ceramic articles comprising the sequential steps of bottom firing said articles to evenly heat each article, top and bottom firing said articles to rapidly heat said articles to a high temperature, and top firing said articles while progressively decreasing said high temperature.

2. A method of firing ceramic articles comprising the sequential steps of bottom firing said articles by conduction and convection to heat said articles from the bottomup, controlling the heat of said bottom firing so that said articles are heated in progressive stages, top-firing said articles by radiant heat while continuing said bottom firing to rapidly raise said articles to a high temperature, soaking said articles at said high temperature, and topfiring said articles by radiant heat and controlling said final step of top-firing so that said articles are cooled in progressive stages.

(References on following page) References Cited by the Examiner UNITED STATES PATENTS 2,974,387 3/1961 Tomkins. 3,050,811 8/ 1962 De Bartolomeis.

OTHER REFERENCES Bennett. Booth- 5 Searle: Encyclopedia of the Ceramic Industries, L0n- Dresslel' 25142 X don, Benn, 1930, vol. 2, page 364. Dressler. Hgrtford et a1. ROBERT F. WHITE, Primary Examiner. Km et WILLIAM J. STEPHENSON, ALEXANDER H. BROD- Dressler. C 10 MERKEL, Examzners.

remer. Criswell 25-153 A. O. MAKI, S. I. COHEN, R. B. MOFFITT, Kupchinsky 25153 Assistant Examiners. 

1. A METHOD OF FIRING CERAMIC ARTICLES COMPRISING THE SEQUENTIAL STEPS OF BOTTOM FIRING ARTICLES COMRPISING THE HEAT EACH ARTICLE, TOP AND BOTTOM FIRING SAID ARTICLES TO RAPIDLY HEAT SAID ARTICLES TO A HIGH TEMPERATURE AND TOP FIRING SAID ARTICLES WHILE PROGRESSIVELY DECREASING SAID HIGH TEMPERATURE. 